The long-term objective of this project is to ascertain the relative roles of cell lineage and cell interactions in the determination of the course of normal and abnormal embryonic development. This project, which uses the embryos of leeches as its working material, is related to health, in that its results are likely to provide insights into the causes of human birth defects. The proposed research plan concerns the embryo of the leech Helobdella triserialis. It addresses the general problem of the commitment to their specific developmental fates of the four bilateral pairs of embryonic blast cell bandlets, designated as n, o, p and q, whose descendant cell clones give rise to the ectodermal tissues of the 32 bilaterally symmetric, serially homologous segments of the adult leech. One main focus of interest is the commitment of cell clones descended from the initially uncommitted, pluripotential primary o and p blast cells to one of two alternative developmental pathways. An empirical test for the commitment of blast cell clones is provided by the phenomenon of "transfating": upon ablation of the p bandlet, an as yet uncommitted o bandlet cell will follow the P developmental pathway, whereas an already committed o bandlet cell will follow the O pathway. The particular developmental pathways taken will be identified by means of cell lineage tracers. The specific aims of this project include a direct demonstration that each of the three successive commitment steps to the O pathway is associated with an asymmetric division in the o blast cell line and that the elements of the fate for which joint commitment occurs in each of the three successive steps are clonally related. The specific aims include also an elucidation of the possible role in the commitment process of gap junctions linking blast cells of adjacent bandlets and of the nature of the pathway guidance which the committed blast cells of the p and q bandlets receive in their centripetal migration from their lateral points of origin in the germinal plate towards the ventral midline, to the positions in which they, or their daughter cells, eventually differentiate into neurons or glia.